The present invention relates to apparatus for assisting or producing ventilation of the lungs of a patient. Such apparatus may broadly be divided into two types, namely internal and external.
Internal ventilator apparatus directs a flow of breathable gas into the lungs of a patient via a face mask or via an intubation. An alternating gas pressure is employed to produce a tidal flow of gas into and out of the patient's lungs.
External ventilator apparatus comprises some form of chest enclosure, which may cover just the chest or even the whole of the patient below the neck. Alternating gas pressure is applied to the interior of the enclosure to produce compression and expansion of the chest and hence to produce a tidal flow of air into and out of the lungs. Generally, no intubation is needed where external ventilation is used.
The application of pressure to the chest by external ventilation can under certain circumstances cause cardiac stimulation of the kind used when resuscitating a patient in cardiac arrest. Also, the induced chest movement may in some cases be physio-therapeutic. External ventilation can be used to produce cardiac stimulation or for physiotherapy, even where there is no clinical need to ventilate the patient. External ventilators to be used for such purposes are included herein within the term "ventilator apparatus".
Many different regimes of alternating pressure have been proposed both for internal and for external ventilation.
Thus, traditionally, internal ventilation involves the application of alternating gas pressure at a frequency similar to that of natural breathing. However, it has been proposed to employ much higher frequencies in so-called high frequency positive pressure ventilation. A drawback of internal ventilation is that it produces a decrease in cardiac output and it needs intubation with its known complications.
External ventilation has also traditionally been carried out at normal breathing frequency, but in EP-A-0192337 there is disclosed a substantially more beneficial regime in which ventilation is carried out using a relatively high frequency alternation of gas pressure about a negative baseline. This technique is proving to have very substantial clinical benefits.